The present invention relates generally to the field of electronic circuits, and more particularly to electronic circuits which include one or more metal-oxide-semiconductor (MOS) devices or other types of field effect devices.
It is important in many electronic circuit applications to provide a mechanism for controlling the transconductance of one or more field effect devices. For example, in amplifiers, buffers, oscillators and other similar circuits, failure to provide proper control of the transconductance of certain MOS devices within the circuits can allow undesirable variations in one or more dimensionless parameters, such as gain, in the presence of variations attributable to factors such as process, temperature, voltage, etc.
Conventional techniques have been unable to provide an adequate solution to the problem of controlling MOS device transconductance, as will be described below.
One such conventional technique involves configuring the circuit so as to maintain the difference between the gate-to-source voltage VGS and the threshold voltage VT of the MOS device substantially equal to the voltage drop across a designated precision resistor. It is generally desirable, however, for the MOS device transconductance control to be implemented in a manner which is substantially independent of the threshold voltage VT.
The issue of independence of VT is addressed in another conventional technique. In accordance with this technique, the circuit is configured such that relative changes in the gate-to-source voltage VGS of the MOS device and the voltage across the precision resistor track one another. This is achieved, for example, by adjusting the amount of the current flowing through the MOS device.
Nonetheless, these and other conventional techniques still suffer from a number of significant drawbacks. For example, the conventional techniques fail to implement the MOS transconductance value gm as the reciprocal of the precision resistor value R. In addition, certain conventional techniques require that particular assumptions be made regarding the relative values of R and gm, such as an assumption that R is much less than 1/gm. Such assumptions are undesirable in that they can unduly limit the level of achievable precision, while also reducing circuit configuration flexibility. Furthermore, certain conventional techniques may require different transistor sizes in order to implement the MOS transconductance control, which further limits achievable precision and configuration flexibility.
As is apparent from the foregoing, a need exists for improved techniques for controlling the transconductance of a MOS device or other field effect device, which address one or more of the drawbacks of the conventional techniques described above.
The present invention provides a transconductance control circuit which in an illustrative embodiment is configured to control the transconductance of at least one MOS device such that it tracks the conductance of a resistor in the presence of circuit variations attributable to factors such as process, temperature or voltage.
In accordance with one aspect of the invention, a transconductance control circuit includes a master device having first and second field effect devices coupled to respective first and second current sources, a reference device coupled to a third current source, and comparison circuitry. The comparison circuitry includes at least first, second and third inputs and at least one output, with the first input configured to receive a reference signal associated with the reference device, the second and third inputs coupled to respective terminals of the first and second field effect devices, and the output coupled to current control inputs of one or more of the current sources.
In an illustrative embodiment, the transconductance control circuit provides a feedback control arrangement in which the comparison circuitry output is utilized to adjust one or more of the current sources such that a difference signal Vg between voltages at the respective terminals of the first and second field effect devices converges to a reference signal VR. As a result, the transconductance gm of the first device converges to the conductance of the reference device.
In accordance with another aspect of the invention, the first field effect device has a transconductance gm and the second field effect device has a transconductance gmt given approximately by             g      m      xe2x80x2        =                  (                  1          +                      α            2                          )            ⁢              g        m              ,
where xcex1 is selected such that xcex1 less than  less than 1.
A transconductance control circuit in accordance with the invention may be implemented, for example, as a portion of an integrated circuit. As a more particular example, the transconductance control circuit may be implemented as a component of an amplifier, buffer, oscillator or other type of electronic circuit which is itself implemented as a portion of an integrated circuit.
Advantageously, the present invention provides a particularly efficient mechanism for controlling the transconductance of a MOS device or other field effect device in the presence of process, temperature or voltage variations, or other types of variations.
The transconductance control circuit in the above-noted illustrative embodiment provides significantly improved precision and flexibility relative to the conventional techniques previously described, in that the MOS transconductance gm is implemented so as to converge to the reciprocal of the resistor value R. Moreover, this transconductance control circuit does not require that any assumptions be made regarding relative resistance and transconductance values, nor does it require different size transistors.